High dynamic range fiber optical link

Abstract

Apparatus, and a corresponding method for its use, for modulating an optical carrier signal with a radio-frequency signal and providing a desirably high dynamic range. An optical signal source is split into a carrier signal and a reference signal, ideally with an optical star coupler, and the carrier signal is phase-modulated with the radio-frequency signal by means of a Pockels cell, producing a phase-modulated signal devoid of even-harmonic distortion, and containing only relatively small odd-harmonic components. In the preferred embodiment of the invention, the phase-modulated signal has its carrier signal suppressed. Then the resulting double-side-band suppressed-carrier signal is combined with the reference signal, preferably with another optical star coupler, to produce the desired modulated optical signal with high dynamic range.

We claim: 
     
       1. A fiber optical communications link, comprising: a source of monochromatic, coherent, optical radiation;   signal dividing means, for producing a reference signal and a carrier signal;   electro-optical phase modulation means, for modulating the carrier signal with a radio-frequency signal to produce a phase-modulated double-sideband signal;   means for removing the carrier signal from the phase-modulated double-sideband signal, to produce a phase-modulated double-sideband suppressed-carrier signal;   means for adjusting the plane of polarization of the phase-modulated double-sideband suppressed-carrier signal to match that of the reference signal; and   signal combining means, for combining the phase-modulated polarization-adjusted double-sideband suppressed-carrier signal with the reference signal, to produce as an output a modulated optical signal with high dynamic range.   
     
     
       2. A fiber optical communication link as set forth in claim 1, wherein: the phase-modulation means includes a Pockels cell.   
     
     
       3. A fiber optical communication link as set forth in claim 1, wherein: the means for removing the carrier signal includes a polarizer.   
     
     
       4. A fiber optical communication link as set forth in claim 1, wherein: the means for adjusting the plane of polarization includes a half-wave plate.   
     
     
       5. A method for modulating an optical carrier signal, comprising the steps of: dividing a coherent source of light into two approximately equal portions, referred to as a reference signal and a carrier signal;   modulating the phase of the carrier signal with a radio-frequency signal;   removing the carrier signal from the phase-modulated signal to provide a phase-modulated double-sideband suppressed-carrier signal;   adjusting the plane of polarization of the phase-modulated double-sideband suppressed-carrier signal to match that of the reference signal;   combining the reference signal and the phasemodulated polarization-adjusted double-sideband suppressed-carrier signal; and   providing, as a result of the combining step, a modulated optical signal that has a high dynamic range.   
     
     
       6. A fiber optical communications link, comprising: a source of monochromatic, electromagnetic radiation;   signal dividing means, for producing a reference signal and a carrier signal;   electro-optical phase modulation means, for modulating the carrier signal with an information signal to produce a phase-modulated double-sideband signal;   means for removing the carrier signal from the phase-modulated double-sideband signal, to produce a phase-modulated double-sideband suppressed-carrier signal;   means for adjusting the plane of polarization of the phase-modulated double-sideband suppressed-carrier signal to match that of the reference signal;   signal combining means, for combining the phase-modulated, polarization-adjusted double-sideband suppressed-carrier signal with the reference signal, to produce as an output a modulated electromagnetic signal with high dynamic range.   
     
     
       7. A method for modulating an optical carrier signal, comprising the steps of: dividing a source of optical radiation which is not limited to the visible spectrum, into two portions, referred to as a reference signal and a carrier signal;   electro-optically modulating the phase of the carrier signal with an information signal to produce a phase-modulated double-sideband signal;   removing the carrier signal from the phase-modulated double-sideband signal to provide a phase-modulated double-sideband suppressed-carrier signal;   adjusting the plane of polarization of the phase-modulated double-sideband suppressed-carrier signal to match that of the reference signal;   combining the reference signal and the phasemodulated polarization-adjusted double-sideband suppressed-carrier signal; and   providing, as a result of the combining step, a modulated electromagnetic signal that has a high dynamic range.